In the beginning, the Earth was pummeled by asteroids. Thousands of these space rocks slammed into the young Earth, sometimes with a force powerful enough to vaporize the seas. They came primarily from the asteroid belt, which was perturbed by the evolving orbits of the Solar System’s larger planets. The evidence of this assault can be seen in the pockmarked surface of the Moon, which also failed to escape the onslaught.
Not long after asteroids stopped raining down, something incredible happened on Earth. Locked in our planet's geological record from the time, is the earliest evidence of life on Earth. For millennia, scientists and philosophers have searched for the answer to the question: Where did life come from?
There are several leading theories of the origin of life on Earth. And one has to do with this violent astronomical era from our planet's past: The Impact-Origin of Life Hypothesis.
Here's everything you need to know about the theory, and why it might just hold true.
What is the Impact-Origin of Life Hypothesis?
The theory is founded on the idea that the cataclysmic assault on the early Earth may have actually been the catalyst necessary to bring life into existence.
Essentially, the force of the asteroids slamming into the Earth would cause the formation of new hydrothermal systems all over the planet. These systems would provide both the venue and the necessary chemistry for life to arise.
David Kring is a senior staff scientist at the Lunar and Planetary Institute of the Universities Space Research Association. He explains that, counter to what you might think, the "very same impact events that made… conditions at the surface completely untenable for life, at the same time [were] constructing these vast subsurface hydrothermal systems."
Basically: While asteroids made Earth's surface unlivable, they were making its subsurface a hotbed of creation.
What is the basis of the Impact-Origin of Life Hypothesis?
Kring is credited with originally proposing the Impact-Origin of Life Hypothesis about 20 years ago, after he and other researchers began to observe evidence of ancient hydrothermal systems in asteroid impact craters.
Around that same time frame, genetic analysis of modern organisms led prominent origins of life researchers to suggest that the earliest life forms on Earth were thermophiles — microbes that thrive in very hot environments.
Early life could have evolved in the subterranean hydrothermal systems where water would be stable and the fledgling organisms would be protected from hostile surface conditions.
It’s been decades since the original RNA studies identified thermophiles as early organisms. Anna-Louise Reysenbach, a professor of microbiology at Portland State University, says that subsequent genetic analysis has strengthened this original claim, but also added some nuance.
“When you look at an evolutionary tree of life, it's starting to change a little,” she says. “[But] it still shows that most of those deep branches are thermophiles.”
How does the Impact-Origin of Life Hypothesis compare to other origin theories?
Sukrit Ranjan, a postdoctoral fellow at Northwestern University who studies astrobiology, says that one of the appealing factors of the Impact-Origin of Life Hypothesis is that it addresses a major issue in understanding the evolution of life on Earth — energy.
Life is thought to have evolved from prebiotic chemistry, the formation of precursor molecules from geological and other passive, non-living processes. At some point, these primitive chemical systems would have had to make the leap to biochemistry — the kinds of reactions that occur in modern organisms.
But many of the chemical reactions that modern life utilizes are not thermodynamically favorable. Essentially, they require extra energy to complete.
“One of the ways they solve that problem is that, well, they're comets, when they hit the Earth, they release a ton of energy. And that energy can go into rearranging molecules into higher energy states,” Ranjan tells Inverse.
“If you have a lot of energy, it can [facilitate] the formation of some of these molecules, which are either implicated in prebiotic chemistry or which are even invoked further down in actual biochemistry," he explains.
What's the evidence?
One of the problems with proving this theory is that the actual craters from the early Earth's days of constant bombardment are long gone, erased by subsequent geological forces.
Instead, some of the best evidence, surprisingly, may be preserved in the Chicxulub crater. Chicxulub, located underneath the northern edge of the Yucatan Peninsula in Mexico, is the impact crater left by the meteor blamed for the death of the dinosaurs 66 million years ago. It is one of the best preserved impact craters on Earth.
Kring and his colleagues recently discovered evidence of an ancient sulfate metabolizing microbial ecosystem that lived in what was once a hydrothermal system beneath the Chicxulub crater. While the impact is much more recent than any that might have catalyzed life on Earth, it can be used as a proxy for studying the geological and biological ramifications of the early Earth asteroid bombardment, Kring says.
The finding doesn’t prove that life evolved in an impact crater, but it is a good sign, Kring says.
“One of the criticisms [of] the Impact-Origin of Life hypothesis is, well, you have this hypothesis, but you've not found microbial ecosystems in these places. And so we've now done that," he says.
To further test the hypothesis, Kring's team are looking for evidence of different kinds of ancient microbes at Chicxulub. Additional lab-based experiments could also help determine if the chemical environments in impact-generated hydrothermal systems could lead to life or biological precursor molecules, Ranjan says.
What are the other theories?
Other possible venues for the origin of life on Earth have also been proposed. These include:
- Deep-sea hydrothermal vents: An attractive prospect for scientists, saltwater reactions create a unique chemical environment that could foster biology springing from chemistry.
- Volcanism: This can generate similar activity to that of the hydrothermal systems created by asteroid impacts.
- Darwin’s original “warm little pond” theory: An oldie but a goodie, this is still in play, but with additional specificity. An area that goes through wet-dry cycles, like a shoreline of a pond, may have helped facilitate the beginning of life.
Researchers have found that exposing cocktails of simple biomolecules to wet-dry cycles in the lab allows them to form larger, more complex molecules necessary for life. Impact-generated hydrothermal systems could also produce alternating wet-dry conditions, Kring says.
“Impact basins would have been partially to wholly filled with water and, thus, had the same attributes of any shoreline community,” he says.
“For the most part, I don't view [these theories] as necessarily contradictory, or like one refuting the other," Ranjan says. "My perspective on this is the problem isn't solved. All of them are worth exploring.”